The reduction of solar heat load entering a building or vehicle through its windows is important in minimum ng air conditioning load and promoting personal comfort. Clear infrared rejecting films have been made using metalized or dyed polymer films and multilayer polymer stacks that reflect or absorb unwanted infrared radiation. Ideally, such films transmit all light in the wavelength region sensitive to the human eye, typically from about 380 to about 700 nanometers (nm), and reject solar radiation outside the visible portion of the spectrum. Metalized and dyed polymer films suffer from reduced performance when used for extended periods of time in window film applications, as they are susceptible to UV degradation and chemical assault from various sources. Their failure mechanism is typically non-uniform, creating poor visual appearance after prolonged exposure. Also, the reflectivity of metal layers originates from a thin coating and if this coating is damaged, the performance of the film is decreased. Clear infrared rejecting film can be made from a quarter wave mirror that has its reflecting band in the near infrared. Infrared rejecting films made from alternating layers of metal oxides have been described in U.S. Pat. No. 5,179,468, U.S. Pat. No. 4,705,356 and EP 0 080 182. Films made from a combination of metal and metal oxide layers have been described in U.S. Pat. Nos. 4,389,452; 4,799,745; 5,071,206; and 5,306,547. Infrared rejecting films made from alternating layers of polymers with high and low indices of refraction have been described in U.S. Pat. Nos. RE 34,605; 5,233, 465; and 5,360,659; U.S. Pat. Nos. 5,882,774 and 08/672,691 entitled "Transparent Multilayer Device; and U.S. Ser. No. 09/006,118 entitled "Multicomponent Optical Body", filed on even date, in which a generalized scheme is described for controlling higher order reflections while maintaining desired relationships between the in-plane and out-of-plane indices of refraction so that the percent reflection of the first order harmonic remains essentially constant, or increases, as a function of incidence angle. These films are not susceptible to the same degradation mechanisms as thin metal or metal oxide layers or dyed films, as it is necessary to destroy the entire film to reduce performance. The films are highly corrosion resistant, have a neutral color, and can have various properties built into the film, such as antistatic, abrasion resistant, and slip layers incorporated in the film's surface. The flexibility and manufacturing cost of the films make them well suited for use as a laminate to glass before window construction as well as for retrofit applications.
For many applications, it is desirable that the infrared reflective film reflect as much solar radiation as possible in the infrared portion of the spectrum while maintaining essentially complete transparency in the visible region of the spectrum. One problem with the quarter-wave polymeric films is that without proper compensation to eliminate overtones, higher order reflections will appear at fractions of the first order reflection and exhibit iridescence and visible color. Mathematically, higher order reflections will appear at EQU .lambda..sub.m =(2/M).times.D.sub.r
where M is the order of the reflection (for example, 2, 3, 4, etc.) and D.sub.r is the optical thickness of an optical repeating unit, of which multiple units are used to form the multilayer stack. Accordingly, D.sub.r is the sum of the optical thicknesses of the individual polymer layers that make up the optical repeating and the optical thickness is the product of n.sub.i, the in plane refractive index of material i, and d.sub.i, the actual thickness of material i. As can be seen, higher order reflections appear at fractions of the first order reflection. For example, a film designed to reflect infrared radiation between about 700 and 2000 nm will also reflect at 1000 nm, 667 nm, 500 nm, of which the latter two are in the visible range and would produce strong iridescent color. It is possible to suppress some higher order reflections by proper selection of the ratio of the optical thicknesses in two component multilayer films. See, Radford et al, "Reflectivity of Iridescent Coextruded Multilayered Plastic Films", Polymer Engineering and Science, vol. 13, No. 3, May 1973. This ratio of optical thicknesses is termed "f-ratio", for a two component film, where f=n1d1/(n1 d1+n2 d2). Such two component films do not suppress successive second, third and fourth order visible wavelengths. Optical coatings comprising layers of three or more materials have been designed which are able to suppress certain higher order reflections. For example, U.S. Pat. No. 3,247,392 describes an optical coating used as a band pass filter reflecting in the infrared and ultraviolet regions of the spectrum. The coating is taught to suppress second and third order reflectance bands, but the materials used in the fabrication of the coating are metal oxide and metal halide dielectric materials which must be deposited in separate steps using expensive vacuum deposition techniques. Other vacuum deposition techniques used to reduce higher order reflections are taught in U.S. Pat. Nos. 3,432,225 and 4,229,066, and in "Design of Three-Layer Equivalent Films", Journal of the Optical Society of America, Vol. 68 (1), 137 (January 1978). U.S Pat. No. RE 34,605 describes an all polymeric three-component optical interference film formed by coextrusion techniques which reflects infrared light while suppressing second, third and fourth order reflections in the visible region of the spectrum. The polymers in the film are required to have closely defined refractive indexes, which limits the choice of polymers which may be used, and production of the film requires separate extruders for each of the three polymeric components. U.S. Pat. No. 5,360,659 describes an all polymeric two-component film which can also be coextruded and reflects infrared light while suppressing second, third, and fourth order wavelengths which occur in the visible portion of the spectrum. The film comprises alternating layers of first (A) and second (B) diverse polymeric materials having a six layer alternating repeat unit with relative optical thicknesses of about 7:1:1:7:1:1 for the layers of A:B:A:B:A:B, respectively. In an alternative embodiment of the invention, the two-component film comprises a first portion of alternating layers comprising the six layer alternating layer repeating unit with relative optical thicknesses of about 7:1:1:7:1:1 for the layers of A:B:A:B:A:B, respectively, and a second portion of alternating layers having a repeating unit AB of equal optical thicknesses.
A second problem with the quarter-wave polymeric films, or any dielectric reflectors, is that the reflection band shifts in wavelength with observed incident angle. When this happens, there is a dramatic color change at high angles of incidence, with cyan color observed in reflection and deep red observed in transmission. The shift in the reflection band is caused by the change in effective index of refraction with angle. Both the band centers and the width of the reflection band change as the incidence angle changes, with the reflecting band always shifting towards shorter wavelengths. This is counterintuitive, as the total path length increases with angle. The band position does not depend on the total path length, but the difference in path length between reflections off the interfaces, and this difference decreases with angle. The high wavelength bandedge also shifts differently from the low wavelength bandedge. For low wavelength bandedges, the change in center and width with angle tend to cancel. For the high wavelength bandedge, the changes add to broaden the band. Typically, for the materials under consideration, a bandedge shifts to about 80% of its normal incidence wavelength when viewed at grazing incidence. For the present applications, in order to not have visible color when the film is viewed at non-normal angles, it is necessary that the low wavelength bandedge of an infrared reflector be positioned sufficiently far into the infrared so that it is not observed at a desired use angle. Typically, the film must be designed so that the short wavelength edge of the normal angle is shifted 100-150 nm away from the edge of the visible spectrum. For example, for a multilayer infrared reflecting film having alternating layers of PEN and PMMA, the short wavelength bandedge must be moved to about 850 nm to eliminate any perceived color with angle. This creates a gap between the edge of the visible spectrum (about 700 nm) and the low wavelength bandedge of about 150 nm.
For many applications it is desirable to reflect as much of the solar spectrum as possible which contributes to heat load, while transmitting all of the visible spectrum. Shifting the bandedge to longer wavelengths for the normal incidence condition results in a gap between the reflectance band and the visible edge of the spectrum, resulting in lower spectral coverage at wavelengths where the solar infrared spectrum is a maximum. This correlates to an over-all increase in the shading coefficient of the film, which is a measure of the amount of solar energy that enters the window compared to that of a simple pane of glass. Accordingly, the need exists for an infrared film that reflects the maximum amount of solar infrared even when the reflecting band is shifted to compensate for visible color when the film is viewed at non-normal angles. It is further needed that such a film be able to eliminate higher order reflections that also contribute to visible color and iridescence.
U.S. Pat. No. 5,486,949 discloses that it may be desirable to incorporate coloring agents such as dyes or pigments into one or more layers of a birefringent polarizer to permit selective absorption of certain wavelengths of light and control the bandwidth of reflected polarized light and the wavelength range of transmitted light U.S. Pat. No. 4,705,356 discloses a thin film optically variable article having substantial color shift with varying angle of light incidence and viewing comprising an optically thick substantially transparent structural element carrying a colorant and a multilayer interference coating, whereby the colorant serves to modify in essentially a subtractive mode the color at normal incidence and the color shift with angle of the multilayer interference coating as seen by transmission of light through the article. Neither U.S. Pat. No. 5,486,949 nor U.S. Pat. No. 4,705,356 disclose an optical body comprising a film having a reflecting band positioned to reflect infrared radiation of at least one polarization at an incident angle normal to the film combined with a component designed to at least partially absorb or reflect infrared radiation at normal incidence in the region resulting from the positioned reflecting band.